1. Field of the Invention
The present invention relates to telecommunication cables, in particular optical fiber cables, comprising elongated elements, in particular buffer tubes, which are capable of blocking a flow of water accidentally penetrated therein.
2. Background Art
International patent application WO 00/21098, in the name of the same Applicant and herein incorporated by reference, discloses elongated solid elements housing at least one optical fiber therein, said elements being made from a water soluble material which, upon being contacted by water, dissolves at least in part and forms a viscous solution of suitable viscosity capable of stopping the longitudinal flow of water along said element. The use of buffer tubes of this kind allows to avoid the use, or at least substantially reduce the amount, of conventional water-blocking means, such as grease-like material, water-swellable powders and the like.
In particular, said element is preferably a buffer tube and is preferably made from a vinyl alcohol/vinyl acetate copolymer (VA-VAc copolymer), generally identified in the art as polyvinylalcohol. These copolymers are generally obtained from partial or complete hydrolysis (i.e. saponification) of the acetate groups of a polyvinyl acetate polymer. Thus, these materials are generally identified by their hydrolysis (or saponification) degree, i.e. the percentage of acetate groups which has been hydrolyzed from the initial vinylacetate polymer. Typically, VA-VAc copolymer having a hydrolysis degree of 98% or higher are considered substantially completely hydrolyzed (or saponified), and are thus referred to as substantially completely hydrolyzed (or saponified) polyvinylalcohol.
As mentioned in WO 00/21098, the water-blocking capacity of the VA-VAc copolymer depends, among other properties, also from the degree of hydrolysis of the material. In particular VA-VAc copolymers completely hydrolyzed are almost insoluble in water, thus being substantially prevented from forming the desired water-blocking viscous solution. Accordingly, WO 00/21098 suggests to employ VA-VAC copolymers having a hydolysis degree of from about 50% to 95%, preferably from 70% to about 90%.
The Applicant has now observed that, while a buffer tube made from said VA-VAC copolymer solves the problem of effectively stopping a flow of water accidentally penetrated inside the cable, its water blocking properties may be impaired upon aging.
In particular the Applicant has observed that, as a consequence of the aging of the material, the water blocking properties of the material can be impaired due to hydrolysis of the acetic groups of the copolymer. In particular, the degree of hydrolysis of the VA-VAc copolymer may increase to such an extent as to severely limit the water blocking properties of the material.
In the art it is known to add compounds (e.g. antioxidants and/or thermal stabilizers) to polymeric compositions, to avoid oxidation and thermal degradation which may occur, for instance, at the high temperatures during the processing of the material, e.g. during the extrusion process. These additives are however generally employed in very limited amounts. For instance, European patent EP 0 458 509 discloses oxidation resistant ethylene/vinyl-alcohol copolymer compositions, having a saponification degree higher than 90%, preferably higher than 95% and comprising 0.01% to 0.5% w/w of a hindered phenolic antioxidant.
The Applicant has now found that the negative aging phenomena of a VA-VAC copolymer can be avoided or at least substantially reduced by adding to said copolymer an effective amount of a hydrolysis stabilizer compound having a specific chelant structure, capable of chelating those site on the VA-VAc copolymer chain which are deemed responsible for the hydrolysis phenomena upon aging.